Nanowire silicon, metalized

C. Li, N. Wang, S. Wong, C. Lee, and S. Lee, Metal silicide/silicon nanowires from metal vapor vacuum arc implantation, Adv. Mater. 14, 218-221 (2002). 

Kim Y, Tsao A, Lee DH, Maboudian R (2012) Solvent-induced formation of unidirectionally curved and tilted Si nanowires during metal-assisted chemical etching. J Mater Chem C 1 220-224 Kiraly B, Yang S, Huang TJ (2013) Multifunctional porous silicon nanopillar arrays antireflection, superhydrophobicity, photoluminescence, and surface-enhanced Raman scattering. Nanotechnology 24 245704... 

In many cases, metal silicides may very well be the catalysts. For example, FeSi2 is being considered to be the catalyst in Fe-assisted nanowire synthesis. This is similar to the silicon mono-oxide case, although it is much easier to understand the mechanisms in the FeSi2 case. It is also possible that during the catalytic processes that silicon diffuses relatively freely through the metal catalyst and consequently, the observed silicides at the end of reaction may be different from those during the catalytic reaction. No direct evidence is available to show whether metal or metal silicide nanoparticles are the tme catalyst. 

Our experiments have shown that hydrogen is critical for growth. The required presence of both hydrogen and metal catalysts, and the virtual absence of silicon vapor suggest that totally new reaction paths assist in the growth of these nanowires under the conditions studied. If silicon does not come directly from the wafer substrate, then it is required to become airborne in some form, as in the CVD production of SiNW, thus enabling tip growth. 

A schematic of the proposed growth model is shown in Fig. 10.23. In this model, Co nanoparticles play a dual catalytic role. On the one hand, they catalyze silane formation by reacting first with silicon to form Co silicides, and then react with hydrogen to form silane while being reduced to Co metal. The second role of Co nanoparticles is their classic catalytic ability of making nanowires by first dissolving the silane and precipitating out Si nanowires. 

In Situ Characterization. As a result of this study, it is clear that Co or other metals may play a dual catal)hic role catalyzing both the growth of nanowires and the production of airborne Si species, such as silane, which acts as a silicon source. An in situ characterization method will eventually be needed. 

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

We have performed ab initio calculations of electronic band structures of nonhydrogenated silicon nanowires in the <001>, <011>. <111> and <112> orientations. Our results clearly indicate that silicon nanowires with the <001>, <111> and <112> axes have turned out to be metallic, while the one with the <011> axis displays the semiconducting behavior. 

Nickel-mesoporous silicon structures are of considerable industrial interest for various applications. Anisotropy of magnetic properties of the nickel nanowires inside porous silicon conditioned by their high aspect ratio is applicable for the magnetic store production [1], Moreover, these structures offer much promise for the rectenna (a special type of antenna that is used to directly convert microwave energy into DC electricity) fabrication. So, it is of value to study in detail the process of the nickel electrodeposition into pores of porous silicon and elaborate control methods for pore filling with metal. 

Si nanowires were first produced using the classical metal catalyst VLS approach [21, 22, 46]. Laser ablation of a metal-containing Si target produces metal/metal silicide nanoparticles that act as the critical catalyst needed for the nucleation of SiNWs. The wires grow further by dissolution of silicon in the metallic nano-cap and concurrent Si segregation from the cap. In a typical experiment, an excimer laser is used to ablate the target placed in an evacuated quartz tube filled with an inert gas, e.g. argon [22]. 

---